For the direct production of tryptophan by fermentation, methods using mutant strains requiring nutrients and/or resistant to tryptophan analogs of the bacteria belonging to the genus Corynebacterium, Brevibacterium, and the like are known (Japanese Patent Publication Nos. 4505/72 and 19037/76).
No example of expressing a desired gene in a host microorganism belonging to the genus Corynebacterium or Brevibacterium by introducing a recombinant DNA containing such desired gene and vector, one of which is foreign to a host microorganism, into such host microorganism has been reported. In the recombinant DNA technology using a microorganism belonging to the genus Corynebacterium or Brevibacterium as a host, it is necessary to construct vectors autonomously replicable in these microorganisms, having selectable markers and useful for cloning of desired genes, and to establish an efficient transformation system. Further, methods to overcome various barriers against the expression of foreign recombinant DNA will be necessary.
The present inventors have constructed plasmid vectors autonomously replicable in a microorganism belonging to the genus Corynebacterium or Brevibacterium and having selectable markers and adequate cloning sites and have developed a highly efficient transformation system (Japanese Published Unexampined Patent Application Nos. 183799/82, 186492/82, 186489/82 and 105999/83). Further, the present inventors have found that the plasmid vectors are useful for expressing a foreign gene in a host micoorganism and increasing the productivity of amino acids by ligating a DNA fragment containing a foreign gene involved in the biosynthesis of amino acids such as glutamic acid and lysine to the plasmid vectors according to the procedures in recombinant DNA technology (U.S. Pat. No. 4,237,224 and Methods in Enzymology 68, Recombinant DNA, edited by Ray Wu, Academic Press 1979) and transforming Corynebacterium glutamicum L-22 or its derivatives using the transformation methods described in Japanese Published Unexamined Patent Application No. 126789/83.
Furthermore, the present inventors have found that a microorganism prepared by the same method has acquired an increased productivity of tryptophan.